1. Field of the Invention
The present invention relates to a vehicle seat which is provided with a seat height adjustment mechanism or what is generally known as “seat lifter mechanism”. In particular, the invention is directed to an improvement of link elements included in such seat lifter mechanism, which is designed to protect the link elements in the case of a rear-end collision.
2. Description of Prior Art
Typically, the seat lifter mechanism includes a pair of front links and a pair of rear links, which such an arrangement that the front links are pivotally connected between the forward underside of a vehicle seat and a floor of vehicle, whereas the rear links are pivotally connected between the rearward underside of the seat and the floor. Vertical synchronized rotation of those four links allows the seat to be adjustably raised and lowered to a desired level or height by means of the associated interlocking mechanisms, parts and control lever, and so forth.
The Japanese Laid-Open Patent Publication No. 2009-286237, or JP 2009-286237 A1 shows the above-stated seat lifter mechanism including two front links and two rear links. According thereto, the two front links are pivotally connected between a forwardly facing portion of seat cushion frame of a vehicle seat and a forwardly facing portion of upper rail of seat slide rail fixed on a floor of vehicle, wherein the upper rail is slidably engaged with a lower rail fixed on the floor. On the other hand, the two rear links are pivotally connected between a rearwardly facing portion of the seat cushion frame and a rearwardly facing portion of the upper rail. A connecting rod is horizontally extended and connected between the two rear links to allow synchronized rotation of both two rear links relative to the axis of the connecting rod in vertical direction. One of the two rear links is formed with a sector gear portion therein. Such sector gear portion is in meshed engagement with a pinion rotatably supported in the seat cushion frame, the pinion being connected to a drive element or a manual operation lever having a brake device incorporated therein. By raising the manual operation lever, the pinion is rotated in normal direction, thereby causing rotation of the sector gear portion, so that both two rear links are rotated upwardly, which in turn causes upward displacement of the seat cushion frame or seat. Reversely, lowering of the manual operation lever causes reverse rotation of the pinion, thus resulting in downward rotation of both two rear links to cause downward displacement of the seat.
In the foregoing conventional seat, as stated above, only one rear link (hereinafter, a first rear link) is provided with the sector gear portion and pivotally connected, via the pinion and brake device, between the seat and floor. But, the other rear link (hereinafter, a second rear link) is not formed as such, but simply pivotally connected between seat and floor. Due to that structure, in the case where a rear-end collision occurs, with a backward great excessive load being applied from a seat occupant under inertia to the seat, the excessive backward load is intensively exerted upon the second rear link, as a result of which, that second rear link is deformed and therefore one lateral side of the seat where the second rear link exists is lowered or inclined downwardly relative to the other lateral side of seat where the first rear link exists. This is because, in the first rear link, the excessive great load is dispersed in the sector gear portion and pinion and thus reduced to avoid deformation of the link, whereas the load is straightly exerted upon the second rear link without being dispersed and reduced, hence deforming the second rear link only. Consequently, in that case, the seating posture of seat occupant becomes unstable, and moreover, both first and second rear links are not smoothly moved in synchronized manner via the connected rod connected therebetween.